1. Field of the Invention
The present invention relates to an exhaust purification apparatus for purifying the exhaust gas of an engine, and more specifically, to an exhaust purification apparatus having an ammonia selective reduction-type NOx catalyst that reduces NOx contained in exhaust gas by using ammonia as a reducing agent, which is produced from the urea-water supplied into the exhaust gas.
2. Description of the Related Art
A conventionally-known exhaust purification apparatus purifies exhaust gas by removing NOx (nitrogen oxides) that is one of pollutants contained in the exhaust gas of an engine. This exhaust purification apparatus includes an ammonia selective reduction-type NOx catalyst (hereinafter, referred to as an SCR catalyst) that is interposed in the exhaust passage of the engine. The SCR catalyst is supplied with ammonia working as a reducing agent. The exhaust gas is purified by reducing NOx with the aid of the ammonia.
In general, such an exhaust purification apparatus supplies urea-water, which is easier than ammonia to use, into the exhaust gas for the purpose of providing ammonia to the SCR catalyst. The urea-water is injected into the exhaust gas by using a urea-water injector or the like. The atomized urea-water that has been supplied from the urea-water injector into the exhaust gas is hydrolyzed by exhaust gas heat. The ammonia produced as a result of the hydrolyzation is supplied to the SCR catalyst. The SCR catalyst adsorb the ammonia supplied to the SCR catalyst, and the SCR catalyst promotes denitrifying reaction between the ammonia and the NOx contained in the exhaust gas. The NOx is thus reduced, and the exhaust gas is purified.
In this process, a portion of the atomized urea-water that has been injected from the urea-water injector is liquefied by colliding with the inner walls of the exhaust passage or other places, and adheres to the exhaust passage, the urea-water injector, etc. The adherent urea-water becomes a solid such as urea crystal (hereinafter, referred to as solid urea) when the moisture contained in the urea-water is vaporized. The solid urea is accumulated on the inner walls of the exhaust passage and the urea-water injector. Due to the vaporization latent heat that is produced during the vaporization of the moisture contained in the adherent urea-water, cold spots are created in places to which the urea-water has adhered. For this reason, more atomized urea-water is prone to be liquefied and adhere to the places to which the urea-water has adhered and the surrounding areas of these places. This encourages the formation of the solid urea.
Such a continuous accumulation of the solid urea might increase exhaust flow resistance in the exhaust passage and block the exhaust passage. Moreover, there is a possibility of causing an operational trouble of the urea-water injector. Unexamined Japanese Patent Publication No. 2005-273503 (hereinafter, referred to as Document 1) suggests an exhaust purification apparatus that removes the solid urea accumulated on a urea-water injector by intermittently injecting the urea-water from the urea-water injector in order to solve the above-mentioned problems. The exhaust purification apparatus described in Document 1 performs the intermittent urea-water injection from the urea-water injector. Consequently, the solid urea accumulated on the urea-water injector is dissolved or blown away and then removed from the urea-water injector.
The exhaust purification apparatus of Document 1, however, removes the solid urea by the intermittent injection when the SCR catalyst has temperature lower than activation temperature, that is, when the SCR catalyst does not need an ammonia supply. The urea-water used for the removal of the solid urea therefore does not contribute to the exhaust purification of the SCR catalyst, so that extra urea-water is consumed.
Furthermore, the exhaust purification apparatus of Document 1 is capable of removing the solid urea accumulated on the urea-water injector by urea-water injection but not removing the solid urea accumulated in the exhaust passage. There causes another problem that the urea-water injected for the removal of the solid urea is liquefied and adheres to the walls of the exhaust passage, which promotes the solid urea accumulation on the walls of the exhaust passage.